No, you can also post 1280x1024, 1152x864, 1024x768, & 800x600. It has to fit into that aspect ratio as all the images here are currently formatted for desktop use. If it doesn't fit into that aspect ratio right now, you can either crop or add some border space to fill it out so that it will fit and you can start sharing.
I have a peice of art here -> http://www.deviantart.com/view/28657919/
That i wish to upload, and it said i cannot. The image is roughly 312kb =)
~Thanks